To give Brad his due, he did add a clause mentioning the sound, but he did miss the point of the original query, which was not "why 'an' rather than 'a'" but "why an indefinite article with a plural noun."  That question has been answered in this thread.

Herb

Herbert F. W. Stahlke, Ph.D.
Emeritus Professor of English
Ball State University
Muncie, IN  47306
[log in to unmask]
________________________________________
From: Assembly for the Teaching of English Grammar [[log in to unmask]] On Behalf Of Richard Wallis [[log in to unmask]]
Sent: January 16, 2009 3:27 PM
To: [log in to unmask]
Subject: "a" and "an"

Our friend Brad tried to summarize the rule as follows:
Use 'a' before a consonant. Use 'an' before a vowel.

Brad’s “simple” rule is actually incorrect, yet it is frequently parroted by teachers and grammarians. It thus continually confuses and trips up non-native speakers because it focuses on spelling rather than phonology.

The correct rule is as follows:
Use “a” before a consonant sound. Use “an” before a vowel sound.

Examples
A useful rule
An unpeeled apple









________________________________
Windows Live™: Keep your life in sync. Check it out.<http://windowslive.com/explore?ocid=TXT_TAGLM_WL_t1_allup_explore_012009> To join or leave this LISTSERV list, please visit the list's web interface at: http://listserv.muohio.edu/archives/ateg.html and select "Join or leave the list"

Visit ATEG's web site at http://ateg.org/

To join or leave this LISTSERV list, please visit the list's web interface at:
     http://listserv.muohio.edu/archives/ateg.html
and select "Join or leave the list"

Visit ATEG's web site at http://ateg.org/